1. Field of the Invention
The present invention relates to a signal reproducing apparatus for reproducing recorded information by irradiating a light spot on a magneto-optical medium and, more particularly, to a signal reproducing apparatus using a magnetic wall movement reproducing method.
2. Related Background Art
Recently, a high-density magneto-optical medium by which information is recorded and reproduced by using a fine light spot has attracted attention. FIG. 1 shows an optical system of a magneto-optical recording/reproducing apparatus for performing recording and reproduction of information with respect to a magneto-optical medium. Referring to FIG. 1, this apparatus includes a semiconductor laser 28 as a light source. Divergent light emitted from the semiconductor laser 28 is collimated into a parallel light beam by a collimator lens 29. This parallel light beam from the collimator lens 29 is fed into an objective lens 32 via a beam shaping prism 30 and a polarizing beam splitter 31 and focused into a fine light spot on a magnetic layer of a magneto-optical medium 33 by the objective lens 32. Meanwhile, an external magnetic field is applied from a magnetic head 34 to the magneto-optical medium 33.
The reflected light from the magneto-optical medium 33 returns to the polarizing beam splitter 31 via the objective lens 32. A portion of the reflected light is separated by the polarizing beam splitter 31 and supplied to a control optical system. The control optical system further separates the separated light beam and supplies one separated light beam to a reproducing optical system 36 to generate an information signal. The control optical system supplies the other separated light beam to a photodetector 44 via a condenser lens 42 and a half prism 43 and to a photodetector 46 via a knife edge 45 to generate a control signal for automatic focusing or automatic tracking. The reproducing optical system 36 includes a halfwave plate 37 for rotating the polarizing direction of a light beam through 45.degree., a condenser lens 38 for focusing the light beam, a polarizing beam splitter 39 for separating the light beam, and photodetectors 40 and 41 for detecting the two light beams separated by the polarizing beam splitter 39. A magneto-optical signal is obtained by differentially detecting signals from the photodetectors 40 and 41.
A method of obtaining the magneto-optical signal will be described below with reference to FIG. 2. First, in the magneto-optical medium 33, information is recorded as a pit (magnetic domain) as a difference between the magnetizing directions. Therefore, when linearly polarized light is given, the polarizing direction of the linearly polarized light rotates clockwise or counterclockwise in accordance with the difference between the magnetizing directions. Assume, for example, that the polarizing direction of linearly polarized light incident on the magneto-optical medium 33 is the direction of a coordinate axis P shown in FIG. 2, reflected light for downward magnetization is R+ which is rotated +.theta.k, and reflected light for upward magnetization is R- which is rotated -.theta.k. When an analyzer is placed in a direction as shown in FIG. 2, light transmitting through the analyzer is A with respect to R+ and B with respect to R-. By detecting these light beams by photodetectors, information can be obtained as the difference between the light intensities. In FIG. 1, the polarizing beam splitter 39 functions as an analyzer; i.e., the polarizing beam splitter 39 is an analyzer in a direction of +45.degree. from the P axis with respect to one separated light beam and an analyzer in a direction of -45.degree. from the P axis with respect to the other separated light beam. That is, the signal components obtained by the photodetectors 40 and 41 have opposite phases. Accordingly, by differentially detecting these signals, a reproduction signal with reduced noise can be obtained.
As described above, in a magneto-optical medium a pit (magnetic domain) as information is recorded as perpendicular magnetization in a thin magnetic film by using thermal energy of a semiconductor laser. This information is read by using a magneto-optical effect. Recently, demands on a higher recording density of this magneto-optical medium have increased. Generally, it can be said that the linear recording density of an optical disk as one magneto-optical medium depends upon the laser wavelength of a reproducing optical system and the NA (Numerical Aperture) of an objective lens. That is, the diameter of a light spot is determined when the laser wavelength .lambda. of a reproducing optical system and the NA of an objective lens are determined. Consequently, the size of a reproducible pit (magnetic domain) has a limitation of about .lambda./(2NA). Therefore, to realize a high density in a conventional optical disk, it is necessary to shorten the laser wavelength of a reproducing optical system or increase the NA of an objective lens. However, improvements of the laser wavelength and the NA of an objective lens also have their limits. Accordingly, development of a technique is being attempted which increases the recording density by improving the construction of a recording medium or a method of reading a recording medium.
For example, Japanese Patent Application Laid-Open No. 3-93058 has proposed a reproducing method in which a signal is recorded in a recording holding layer of a multilayered film having a reproduction layer and the recording holding layer which are magnetically coupled with each other and, after the directions of magnetization are aligned, laser light is irradiated on the reproduction layer to heat the reproduction layer, thereby reading the signal while transferring the signal recorded in the recording holding layer to the heated area in the reproduction layer. Also, Japanese Patent Application Laid-Open No. 6-290496 has proposed a magnetic wall movement reproducing method in which a light spot is irradiated on a magneto-optical medium, which is formed by stacking a plurality of magnetic layers, to transfer a pit (magnetic domain) recorded as perpendicular magnetization in a recording layer to a reproduction layer, and magnetic walls of the pit (magnetic domain) transferred to the reproduction layer are moved to make this pit (magnetic domain) larger than the pit (magnetic domain) in the recording layer, thereby reproducing the pit.
This magnetic wall movement reproducing method will be described below. FIGS. 3A to 3D are schematic views for explaining a magneto-optical medium used in the magnetic wall movement reproducing method and the action of the magneto-optical medium. FIG. 3A is a schematic view showing the surface of the magneto-optical medium. FIG. 3B is a schematic view showing the section of the magneto-optical medium. Referring to FIGS. 3A and 3B, a reproducing light spot 48 and an information track 47 on the magneto-optical medium are shown. The magneto-optical medium is constituted by three magnetic layers, i.e., first, second, and third magnetic layers 50, 51, and 52. Arrows in each layer indicate the directions of atomic spins. Magnetic walls 49 are formed in regions where the directions of spins are opposite to each other.
FIG. 3C is a graph showing a temperature distribution formed in this magneto-optical medium. Assume that in a position X.sub.s the medium temperature is a temperature T.sub.s near the Curie temperature of the second magnetic layer 51. FIG. 3D shows the distribution, which corresponds to the temperature distribution in FIG. 3C, of a magnetic energy density .sigma.1 in the first magnetic layer 50. As shown in FIG. 3D, when a gradient of the magnetic wall energy density .sigma.1 exists in an X direction, a force F1 is produced with respect to magnetic walls present in a position X in the individual layers. This force F1 so acts as to move the magnetic walls to a portion where the magnetic wall energy is low. In the first magnetic layer 50, magnetic wall coercivity is small, and the magnetic wall mobility is large. Therefore, the magnetic walls in the first magnetic layer 50 alone are easily moved by the force F1. However, the medium temperature is still lower than T.sub.s in a region before the position X.sub.s (on the right-hand side in FIG. 3D). Accordingly, by exchange coupling with the third magnetic layer 52 having a large magnetic wall coercivity, magnetic walls in the first magnetic layer 50 are fixed to positions corresponding to the positions of magnetic walls in the third magnetic layer 52.
If one of the magnetic walls 49 exists in the position X.sub.s of the medium as shown in FIG. 3B, the medium temperature rises to the temperature T.sub.s near the Curie temperature of the second magnetic layer 51, and this breaks the exchange coupling between the first and third magnetic layers 50 and 52. As a consequence, the magnetic wall 49 in the first magnetic layer 40 instantaneously moves, as indicated by an arrow, to a region where the temperature is higher and the magnetic wall energy density is lower. That is, when the reproducing light spot 48 passes by, the magnetic wall moves as described above, and atomic spins in the first magnetic layer 50 in the spot are pointed in the same direction. The magnetic wall instantaneously moves as the medium moves, and all atomic spins in the light spot are reversed and pointed in the same direction. Consequently, a signal reproduced by the light spot always has a fixed amplitude regardless of the size of a pit (magnetic domain) recorded in the third magnetic layer 52; i.e., the signal is free from the problem of waveform interference resulting from optical diffraction limits. Accordingly, it is possible to reproduce a pit (magnetic domain) smaller than about .lambda./(2NA) which is the resolution limit determined by the laser wavelength .lambda. and the NA of an objective lens. Consequently, the recording density can be increased.
FIG. 4 is a schematic view showing the arrangement of an optical system used in magnetic wall movement reproduction. Referring to FIG. 4, a recording/reproducing semiconductor laser 53 has a wavelength of, e.g., 780 nm. A heating semiconductor laser 55 has a wavelength of, e.g., 1.3 .mu.m. These semiconductor lasers 53 and 55 are so arranged that their laser beams are incident as P-polarized light on a recording medium. The laser beams emitted from the semiconductor lasers 53 and 55 are shaped into substantially circular beams by beam shaping means (not shown) and converted into parallel light beams by collimator lenses 54 and 56. This optical system further comprises a dichroic mirror 57 and a polarizing beam splitter 58. The dichroic mirror 57 is so designed as to transmit 100% of light with a wavelength of 780 nm and reflect 100% of light with a wavelength of 1.3 .mu.m. The polarizing beam splitter 58 transmits 70 to 80% of P-polarized light and reflects almost 100% of S-polarized light as a vertical component.
The parallel light beams converted by the collimator lenses 54 and 56 enter an objective lens 59 via the dichroic mirror 57 and the polarizing beam splitter 58. This part of the optical system is so designed that a light beam of 780 nm becomes large with respect to the aperture of the objective lens 59 and a light beam of 1.3 .mu.m becomes small with respect to the aperture of the objective lens 59. Accordingly, even when the same objective lens 59 is used, the action of the NA of the lens is small to the 1.3-.mu.m light beam, so the size of a light spot on a recording medium 60 becomes larger than that formed by the 780-nm light beam. The reflected light from the recording medium 60 is formed into a parallel light beam through the objective lens 59 and reflected by the polarizing beam splitter 58 to form a light beam 61. This light beam 61 is incident on an optical system (not shown) and subjected to, e.g., wavelength separation, thereby generating a servo error signal or an information reproduction signal.
The relationship between the recording/reproducing light spot and the heating light spot on the recording medium shown in FIG. 4 will be described below with reference to FIGS. 5A and 5B. Referring to FIG. 5A, a recording/reproducing light spot 62 has a wavelength of 780 nm, and a heating light spot 63 has a wavelength of 1.3 .mu.m. Pits (magnetic domains) recorded in a land 65 have magnetic walls 64, and grooves 66 are also formed. A region 67 is a heated region whose temperature is raised by the heating light spot 63. In this manner, by coupling the recording/reproducing light spot 62 and the heating light spot 63 on the land 65 between the grooves 66, a temperature gradient as shown in FIG. 5B can be formed on the moving medium. The relationship between the temperature gradient and the recording/reproducing light spot 62 is as explained in FIGS. 3A to 3D. Consequently, magnetic wall movement reproduction as described above can be performed.
In the above conventional magnetic wall movement reproducing method, reproduction is performed by using a reproducing light spot and a heating light spot. This increases the number of parts such as semiconductor lasers and complicates the structure. Therefore, it is possible to simplify the structure by performing magnetic wall movement reproduction by using a reproducing light spot alone without using any heating light spot. However, when reproduction is thus performed only with a reproducing light spot, the peak of a high-temperature portion on a magneto-optical medium comes inside the reproducing light spot. Accordingly, magnetic walls move in a direction opposite to the moving direction of the magneto-optical medium and in the same direction as the moving direction of the magneto-optical medium. Consequently, the influences of the two signals mix in the reproduction signal, and this makes the information difficult to reproduce.